Wavelength selecting method, position detecting method and apparatus, exposure method and apparatus, and device manufacturing method

ABSTRACT

A wavelength selecting method for selecting a wavelength of light, the light being used to detect a position of a target with a signal from an image of an alignment mark covered with resist, includes the steps of obtaining a reflectance of the resist at a position outside the alignment mark by irradiating lights having plural wavelengths to the resist at the position, and selecting one of the lights which one has a wavelength that provides the maximum value of  reflectance among the reflectances measured by the measuring step or which one has a wavelength that falls within a predetermined wave range centering on the wavelength that provide the maximum reflectance.

BACKGROUND OF THE INVENTION

The present invention relates generally to position detecting apparatusand method, and more particularly to position detecting apparatus andmethod for detecting a position of an object, such as a wafer, in anexposure apparatus that manufactures various devices includingsemiconductor chips such as ICs and LSIS, CCDs, and magnetic heads. Thepresent invention is suitable, for example, for an alignment between areticle and a wafer.

Recent demands for smaller and thinner electronic apparatuses haveincreasingly required finer processing to semiconductor devices mountedon these electronic apparatuses. A reduction projection exposureapparatus has been conventionally employed which uses a projectionoptical system to project a circuit pattern formed on a mask (or areticle) onto a wafer, etc. to transfer the circuit pattern, inphotolithography technology for manufacturing the semiconductor device.

The minimum critical dimension (“CD”) to be transferred by theprojection exposure apparatus or resolution is proportionate to awavelength of light used for exposure, and inversely proportionate tothe numerical aperture (“NA”) of the projection optical system. Theshorter the wavelength is, the better the resolution is. Therefore,recent demands for finer processing to semiconductor devices havepromoted use of a shorter wavelength of the UV light from an ultra-highpressure mercury lamp (i-line with a wavelength of about 365 nm) to KrFexcimer laser (with a wavelength of about 248 nm) and ArF excimer laser(with a wavelength of about 193 nm). More recently, the light having ashorter wavelength is reduced to practice, such as F₂ laser (having awavelength about 157 nm) as ultraviolet (“UV”) light having a shorterwavelength and the extreme ultraviolet (“EUV”) light.

In order to satisfy these requirements, a step-and-repeat exposureapparatus (also referred to as a “stepper”) for projecting and exposingan approximately square exposure area all together onto a wafer with areduced exposure area has been replaced mainly with a step-and-scanexposure apparatus (also referred to as a “scanner”) for accuratelyexposing a wide screen of exposure area through a rectangular or arcslit with relatively and quickly scanning the reticle and the wafer.

In exposure, the scanner uses a surface position detector in an obliquelight projection system to measure a height (or a surface position) of acertain position on the wafer before the exposure slit area reaches thecertain position on the wafer, and correctly positions the wafer surfaceat the best focus position when exposing the certain position.

In particular, the exposure slit area has plural measurement points inlongitudinal direction of the exposure slit, i.e., a directionperpendicular to the scan direction, to measure an inclination (tilt) ofthe surface as well as a height (focus) of the wafer's surface position.Japanese Patent Application, Publication No. 6-260391 (corresponding toU.S. Pat. No. 5,448,332) discloses, for example, a method for measuringthe focus and tilt.

The projection exposure apparatus requires a highly precise alignmentthat aligns positions between the reticle and the wafer along with thefine processing to the circuit pattern (or the improvement of theresolution). The necessary precision for the alignment is generallyabout ⅓ of a circuit critical dimension and, for example, which is 60 nmthat is ⅓ as long as the current design width of 180 nm.

In the alignment between the reticle and the wafer in each shot,positions of alignment marks that are exposed on the wafer for each shotare detected simultaneous with a detection of a circuit pattern on thereticle. The position of the alignment mark is detected by receiving thelight from the alignment mark via an optical system and a CCD camera,signal-processing the obtained electric signal using various parameters,and positioning the wafer relative to the reticle based on the detectionresults. See, for example, Japanese Patent Application, Publication No.6-260390 (corresponding to U.S. Pat. No. 5,703,685). The detectionresults include, for example, a wafer's magnification, a wafer'srotation, a shift amount, etc.

The recent, increasingly shortened wavelength of the exposure light andthe higher NA of the projection optical system have required anextremely small depth of focus (“DOF”) and stricter accuracy with whichthe wafer surface to be exposed is aligned to the best focus position.This accuracy is also referred to as the focus accuracy. A measurementerror of a surface position detecting means cannot become negligible;the measurement error results from influence of a pattern on a wafer andan uneven coating thickness of the resist applied to the wafer.

For example, the resist's uneven coating thickness generates a step neara peripheral circuit pattern and a scribe line. The influence of theresist's uneven coating thickness is smaller than the DOF but issignificant to the focus measurement. This step enlarges an inclinedangle on a resist surface, and causes the reflected light detected bythe surface position detecting means to offset from a regular reflectionangle due to the reflections and refractions. The pattern density on thewafer causes a reflectance difference between a pattern crowded area anda coarse area. Since the reflected light detected by the surfaceposition detecting means changes the reflecting angle and intensity, thedetected waveform obtained from this reflected light contains anasymmetry that causes a measurement error, and a surface position on thewafer cannot be detected due to the remarkably lowered contrast in thedetected waveform.

In general, the wafer process causes non-uniform pattern steps on awafer surface and the uneven resist's coating thickness, resulting inthe poor reproducibility in one wafer and among wafers and making theoffset process difficult. This causes the exposure to stop since thewafer's surface position cannot be measured during the exposure, and thelarge defocus results in defective chips or lowered yield.

It is vital for the present semiconductor industry to improve theoverlay accuracy of the wafer for high semiconductor device performanceand manufacture yield (or throughput). However, the wafer induced shift(“WIS”) causes a large error (an asymmetry of alignment signal) in adetection result, and deteriorates the alignment accuracy. The factorsinclude an asymmetry of an alignment mark and an asymmetry of theresist.

Various methods are proposed as measures for the alignment mark'sasymmetry: For example, one method (1) maintains a wide illuminationwavelength width for reduced influence of the thin film interference onthe alignment mark. Another method (2) evaluates an exposure resultafter the alignment using prior several (send-a-head) wafers, andexposes subsequent wafers by feeding back the alignment offset amount tothe exposure apparatus. Still another alignment method (3) uses awavelength that irradiates plural lights having different wavelengths,and selects, for the alignment, one of the lights whose wavelengthprovides the maximum contrast in the alignment signal.

However, in order for the method (1) to completely eliminate theinfluence of the thin film interference of the resist, the method (1)requires a wavelength width twice as wide as the current illuminationwavelength width of about 200 nm for the future thinning resistthickness, such as 200 nm to 300 nm. While the alignment optical systemshould correct the chromatic aberration for the wavelength width, it istechnically very difficult to simultaneously correct the imagingpositions and imaging magnifications for respective wavelengths.Therefore, the method (1) is unlikely to be viable.

The method (2) needs non-product wafers (“NPW”) for offset calculations,and deteriorates the cost performance of the device manufacture becausea 300 mm wafer that has already reduced to practice is so expensive persheet. Moreover, the fluctuation of manufacture process varies steps andwidth of the alignment mark, and causes scattering errors that cannot beeliminated by the alignment offset.

The method (3) is likely to be implemented without increasing the NPW.However, the wavelength that provides the highest contrast in thealignment signal is not always closest to the wavelength that providesthe least asymmetric errors of the alignment mark. Therefore, thealignment that uses the light having the wavelength that maximizes thecontrast in the alignment signal does not always minimize the errorscaused by the asymmetry of the alignment mark.

There are demands for position detecting method and apparatus and anexposure apparatus, which reduce the influence caused by the asymmetryof the alignment mark and provide highly precise position detections. Inaddition, there are other demands for an exposure method and apparatus,which realize a high focus accuracy relative to a small DOF, and improvethe yield.

BRIEF SUMMARY OF THE INVENTION

A wavelength selecting method according to one aspect of the presentinvention for selecting a wavelength of light, the light being used todetect a position of a target with a signal from an image of analignment mark covered with resist, includes the steps of obtaining areflectance of the resist at a position outside the alignment mark byirradiating lights having plural wavelengths to the resist at theposition, and selecting one of the lights which one has a wavelengththat provides the maximum value of

reflectance among the reflectances measured by the measuring step orwhich one has a wavelength that falls within a predetermined wave rangecentering on the wavelength that provide the maximum reflectance.

A wavelength selecting method according to another aspect of the presentinvention for selecting a wavelength of light, the light being used todetect a position of a target with a signal from an image of analignment mark covered with resist, includes the steps of previouslystoring, for each optical constant and coating thickness of the resist,a wavelength of light that maximizes a reflectance of the resist at aposition outside the alignment mark, obtaining the optical constant andcoating thickness of the resist, and selecting one of the wavelengthswhich one maximizes the reflectance of the resist and corresponds to theoptical constant and coating thickness of the resist obtained by theobtaining step.

A position detecting method according to still another aspect of thepresent invention for detecting a position of a target using a signalfrom an image of an alignment mark coated with a resist, includes thesteps of irradiating an alignment mark using light having a wavelengthselected by using the above wavelength selecting method, and generatingthe signal from the light reflected from the alignment mark irradiatedby the irradiating step.

A position detecting apparatus according to still another aspect of thepresent invention for detecting a position of a target using a signalfrom an image of an alignment mark formed on the target that is coatedwith resist, includes a selector for selecting the light having awavelength that maximizes a reflectance of the resist at the positionoutside the alignment mark, and a signal processor for determining aposition of the alignment mark relative to the signal generated from thelight having the wavelength selected by the selector.

An exposure apparatus according to another aspect of the presentinvention for exposing a pattern on a reticle onto an object via aprojection optical system, the exposure apparatus comprising the aboveposition detecting apparatus, and using the position detecting apparatusfor alignment of the object.

An exposure method according to another aspect of the present inventionfor exposing a pattern on a reticle onto an object via a projectionoptical system, includes the steps of aligning the object using lighthaving a wavelength selected using the above wavelength selectingmethod, and projecting the pattern onto the object that has beenaligned.

An exposure apparatus according to another aspect of the presentinvention for exposing a pattern on a reticle onto an object via aprojection optical system, includes an irradiation unit for irradiatingplural lights having different wavelengths onto two or more measuringpoints in each of the plural shots, a detector for detecting reflectedlight from the measuring points, a selector for selecting one of thewavelengths in accordance with the measuring positions of the pluralshorts based on a detection result from the detector, and a calculatorfor calculating a position of each of the plural shots in anoptical-axis direction based on the one of the wavelengths selected bythe selector.

A wavelength selecting-method according to another aspect of the presentinvention for selecting a wavelength of light used to detect a positionof a target, includes the steps of projecting an image of a pattern thatincludes plural elements using plural lights having differentwavelengths onto measuring positions on plural areas of the target, anddetecting a signal waveform from plural areas of the target, obtainingplural element intervals for each of the plural wavelengths based on thesignal waveform detected by the detecting step, calculating a standarddeviation of the plural element intervals at the measuring positions inthe plural areas based on the plural element intervals, and selectingone of the wavelengths which one provides a minimum standarddistribution calculated by the calculating step.

A wavelength selecting method for selecting a wavelength of light usedto detect a position of a target, includes the steps of projecting animage of a pattern that includes plural elements using plural lightshaving different wavelengths onto measuring positions on plural areas ofthe target, and detecting a signal waveform from plural areas of thetarget, obtaining a signal contrast of the signal waveform for each ofthe plural wavelengths based on the signal waveform detected by thedetecting step, and selecting one of the wavelengths which one providesa maximum contrast of the signal waveform for each of the pluralwavelengths based on the signal waveform detected by the detecting step.

A wavelength selecting method for selecting a wavelength of light usedto detect a position of a target, includes the steps of projecting animage of a pattern that includes plural elements using plural lightshaving different wavelengths onto measuring positions on plural areas ofthe target, and detecting a signal waveform from plural areas of thetarget, calculating a reflectance at the measuring position of lightthat passes the element based on the signal waveform detected by thedetecting step, and selecting one of the wavelengths which one providesa maximum reflectance of the signal waveform for each of the pluralwavelengths based on the signal waveform detected by the detecting step.

A wavelength selecting method for selecting a wavelength of light usedto detect a position of a target, includes the steps of projecting animage of a pattern that includes plural elements using plural lightshaving different wavelengths onto measuring positions on plural areas ofthe target, and obtaining positional information indicative of theposition, obtaining an approximate curved surface from an average of thewavelengths of the positional information obtained in the obtainingstep, calculating an offset amount of the positional information fromthe approximate curved surface, and selecting one of the wavelengthswhich one minimizes scattering among the plural areas based on theoffset amounts calculated by the calculating step.

An exposure method for exposing a pattern on a reticle onto plural shotson an object through a projection optical system, includes the steps ofdetecting a position of the object in an optical-axis direction usinglight having a wavelength selected by the above wavelength selectingmethod, and scanning the object in synchronization with the reticle,based on the position of the object in the optical-axis directiondetected by the detecting step.

An exposure method for exposing a pattern on a reticle onto plural shotson an object through a projection optical system, includes the steps ofdetecting a position of the reticle in an optical-axis direction usinglight having a wavelength selected by the wavelength selecting method,and scanning the reticle in synchronization with the object, based onthe position of the reticle in the optical-axis direction detected bythe detecting step.

A device manufacturing method according to another aspect of the presentinvention includes the steps of exposing an object using the aboveexposure apparatus, and developing the object that has been exposed.

Other objects and further features of the present invention will becomereadily apparent from the following description of the preferredembodiments with reference to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a shape of an alignment mark.

FIG. 2 is a view showing a simulation result of images of alignmentmarks AM shown in FIG. 1.

FIG. 3 is a view showing a relationship between an alignment error and acontrast of an alignment signal relative to a wavelength of illuminatingalignment light.

FIG. 4 is a block diagram showing a structure of an exposure apparatusaccording to one aspect of the present invention.

FIG. 5 is a block diagram showing principal components in an alignmentoptical system shown in FIG. 4.

FIGS. 6A and 6B are views of one exemplary shape of an alignment mark.

FIG. 7 is a view showing a typical optical detection result of analignment mark shown in FIG. 6.

FIGS. 8A, 8B and 8C are views for explaining a template matching methodapplicable to the detection result shown in FIG. 7.

FIG. 9 is a view showing that a shot arrangement is offset on a waferrelative to a XY coordinate system on a wafer stage of the exposureapparatus shown in FIG. 4.

FIG. 10 is a vector diagram that shows a primary coordinate conversionexpressed by Equation 6.

FIG. 11 is a plane view showing one exemplary structure of a wavelengthselecting means in an alignment optical system shown in FIG. 5.

FIG. 12 is a spectral characteristic of a band-pass interference filterin the wavelength selecting means shown in FIG. 11.

FIG. 13 is a sectional view showing a shape of an alignment mark.

FIG. 14A is a view showing a relationship between a wavelength of thealignment light and an alignment error, and FIG. 14B is a view showing aspectral characteristic of a reflectance at a non-mark part shown inFIG. 13.

FIG. 15 is a block diagram showing one exemplary structure of areflectance measuring unit.

FIG. 16 is a view showing refractive indexes of a wafer and a resist anda coating thickness of the resist.

FIG. 17 is a view showing a relationship among the reflectance relativeto a wavelength of the alignment light, a contrast of an alignmentsignal, and an alignment error.

FIG. 18 is a block diagram of principal components in the alignmentoptical system.

FIG. 19 is a view showing a spectral reflectance characteristic of adichroic mirror shown in FIG. 18.

FIG. 20 is a block diagram of principal components in the alignmentoptical system as a variation of the alignment optical system shown inFIG. 18.

FIG. 21 is a schematic sectional view of a structure of a band-passfilter in the alignment optical system shown in FIG. 20.

FIG. 22 is a plane view showing an image of an alignment mark on an areasensor in the alignment optical system shown in FIG. 20.

FIG. 23 is a block diagram of principal components in an alignmentoptical system as a variation of the alignment optical system shown inFIG. 20.

FIG. 24 is a sectional view showing a spectral operation of adiffraction grating in the alignment optical system shown in FIG. 23.

FIG. 25 is a flowchart for explaining a method for fabricating devices(semiconductor chips such as ICs, LSIs, and the like, LCDs, CCDs, etc.).

FIG. 26 is a detailed flowchart for Step 4 of wafer process shown inFIG. 25.

FIG. 27 is a schematic block diagram showing a structure of an exposureapparatus according to another aspect of the present invention.

FIG. 28 is an enlarged block diagram showing a structure of a focus 1tilt detecting system.

FIG. 29 is a plane view of one exemplary pattern plate shown in FIG. 28.

FIGS. 30A and 30B show a signal waveform detected by a detector shown inFIG. 28 when the pattern plate shown in FIG. 29 is used.

FIG. 31 is a view for explaining a reflectance difference resulting froman uneven coating thickness of a resist and the wafer's pattern step.

FIG. 32 is a view for explaining a reflectance difference between anarea having a small step pattern density on a wafer and an area having alarge step pattern density.

FIG. 33 is a graph showing a wavelength dependency of a reflectance of awafer relative to the resist's coating thickness.

FIG. 34 is a plane view showing one exemplary arrangement of measuringpoints on the wafer.

FIG. 35 is a plane view showing a shot layout on the wafer.

FIG. 36 is a plane view showing one exemplary arrangement of measuringpoints in a slit of the wafer.

FIG. 37 is a plane view showing one exemplary arrangement of measuringpoints in a slit of the wafer.

FIG. 38 is a flowchart for explaining an optimal wavelength selectingmethod of light irradiated by the focus/tilt detecting system.

FIG. 39 is a schematic perspective view showing an exposure area, andfocus and tilt measuring positions on the wafer.

FIG. 40 is a flowchart for explaining an exposure method using theexposure apparatus shown in FIG. 27.

FIG. 41 is a block diagram showing a structure of a variation of thefocus/tilt detecting system shown in FIG. 28.

FIG. 42 is a plane view showing a pattern plate as a variation of thepattern plate shown in FIG. 29.

FIG. 43 is a flowchart for explaining an optimal wavelength selectingmethod of light irradiated by the focus/tilt detecting system when thepattern plate shown in FIG. 42 is used.

FIG. 44 is a signal waveform diagram detected by the detector shown inFIG. 28 when the pattern plate shown in FIG. 42 is used.

FIG. 45 is a plane view showing a pattern plate as a variation of thepattern plate shown in FIG. 42.

FIGS. 46A and 46B are a signal waveform diagram detected by the detectorshown in FIG. 28 when the pattern plate shown in FIG. 45 is used.

FIG. 47 is a flowchart for explaining an optimal wavelength selectingmethod of light irradiated by the focus/tilt detecting system.

FIG. 48 shows an approximate curved surface of an A-A′ surface shape onthe wafer shown in FIG. 35 and a graph that plots surface positionmeasurement values relative to sample shots for each wavelength.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the instant inventors have eagerly reviewed a relationshipbetween a wavelength that provides the highest contrast in the alignmentsignal and a wavelength that provides the least asymmetric errors in thealignment mark, in order to provide the position detecting method andapparatus and exposure apparatus, which reduce the influence caused bythe asymmetry of the alignment mark and provide highly precise positiondetections.

The alignment mark's measurement errors (“alignment errors”) due to theasymmetry of the uneven coating of the resist on the alignment mark andthe contrast of the alignment signal are analyzed with a model shown inFIG. 1. FIG. 1 is a sectional view of alignment marks AM, and resist PRis applied onto the alignment mark AM. When the resist PR is applied ona wafer WP by the spin coat, the resist PR radially flows from thecenter to the outer circumference and produces uneven coating near thesteps of the alignment marks AM as shown in FIG. 1. The uneven coatingforms a concave apart from the alignment mark AM by a distance sf.

FIG. 2 shows a simulation result of the images of the alignment marks AMshown in FIG. 1. Referring to FIG. 2, the contrast of the alignmentsignal varies according to a wavelength of the alignment light thatilluminates the alignment mark AM.

FIG. 3 is a view showing a relationship between the contrast of thealignment signal relative to the wavelength of the illuminatingalignment light, and the mark position calculated from the alignmentsignal. The mark position is based on a reference mark position in acase where there is no influence of the asymmetric component, and thealignment error is defined as an offset amount from the reference markposition. Referring to FIG. 3, when the alignment light has a wavelengthof 540 nm, the contrast becomes maximum but the alignment error does notbecome zero; when the alignment light has a wavelength of 580 nm, thealignment error becomes zero. From this result, the instant inventorshave discovered that the alignment light having the wavelength thatmaximizes the contrast does not always minimize the alignment error.

A description will now be given of preferred embodiments of the presentinvention, with reference to the accompanying drawings. The same elementin each figure is designated by the same reference numeral, thus adescription thereof will be omitted. FIG. 4 is a schematic view of anexposure apparatus 1 that uses an inventive position detecting method,and omits an illumination apparatus for illuminating the reticle RC, onwhich a circuit pattern is formed. The exposure apparatus 1 is aprojection exposure apparatus that exposes onto a wafer the circuitpattern on a reticle, e.g., in a step-and-repeat or a step-and-scanmanner. The exposure apparatus 1 includes, as shown in FIG. 4, aprojection optical system 10 for projecting a reduced size of a reticleRC that has a desired pattern (such as a circuit pattern), a wafer chuck25 that holds a wafer 100, onto which a primary coat pattern andalignment marks 110 have been formed in a pretreatment step, a waferstage 20 that positions the wafer 100 at a predetermined position, analignment optical system 200 that measures a position of the alignmentmark 110 on the wafer 100, an alignment signal processor 30, and acontroller 40. The alignment optical system 200, the alignment signalprocessor 30 and the controller 40 serve cooperatively as a positiondetecting apparatus for detecting a position of the wafer 100.

The wafer 100 is an object to be exposed, onto which the alignment mark110 shown in FIG. 6 is formed, and a photoresist is applied. The wafer100 broadly covers a liquid crystal substrate and other objects to beexposed.

The wafer stage 20 supports the wafer 100 via the wafer chuck 25. Thewafer stage 20 can apply any structure known in the art, and a detaileddescription of the structure and operation will be omitted.

The alignment optical system 200 detects the alignment mark 110 on thewafer 100, and measures a position of the wafer 100. FIG. 5 is a blockdiagram of principal components of the alignment optical system 200shown in FIG. 4. FIG. 5 shows an exemplary optical system for detectinga position of the wafer 100 in an X direction. An optical system fordetecting a position in a Y direction may use that for the x directionrotated by 90° around a z axis. The alignment mark 110 may use the markfor the x direction rotated by 90° around the x axis. A description willbe given of an optical system for detecting a position of the wafer 100in the x direction.

The alignment optical system 200 includes an illumination optical system210, and an imaging optical system 220. A lens 212 enlarges andcollimates illumination light (or alignment light) from an alignmentlight source 211, and the resultant light passes a wavelength selectingmeans 250 that selectively transmits an arbitrary wavelength. Then, alens 213 condenses the resultant light again. During this period, avariable aperture stop 214 adjusts the coherency or coherence factor (a)of the illumination light. An aperture 215 is located at a positionconjugate with the wafer 100, and serves as a field stop for preventingunnecessary light from illuminating an area around the alignment mark110 on the wafer 100.

The light condensed by the lens 213 is collimated by a lens 216,reflected by a beam splitter 221, passes through a lens 222, andilluminates the alignment mark 110 on the wafer 100. The light reflectedfrom the alignment mark 110 passes through the lens 222, beam splitter221, lenses 223, 224 and 225, and is received by a line sensor 226. Avariable aperture stop 227 can adjust a numerical aperture (“NA”) in theimaging optical system 220. The alignment mark 110 is imaged on the linesensor 226 while enlarged at an imaging magnification of about 100times. The line sensor 226 can use a two-dimensional area sensor. Use ofthe area sensor enables the alignment marks for X and Y directions to bedetected.

The alignment marks 110 are arranged on a scribe line for each shot.FIGS. 6A and 6B are plane and sectional views of one exemplary shape ofthe alignment mark 110. The alignment mark 110 arranges, as shown inFIG. 6, four rectangular mark elements 182A at a pitch of 20 μm in the Xdirection as a measuring direction, each of which has a size of 4 μm inthe X direction and 20 μm in the non-measurement direction Y. The markelement 112 has a concave sectional shape that is formed by etching andhas a rectangular contour defined by a line width (critical dimension)of 0.6 μm, as shown in FIG. 6B. Although the resist is applied to theactual alignment mark 110, FIG. 6 omits the resist.

When the alignment mark 110 shown in FIG. 6 is used, the line sensor 226generally takes an image as shown in FIG. 7, due to a generation andinterference of scattered rays at a lens' edge outside a NA of thelenses 222 to 225 in the alignment optical system 200. The alignmentmark 110 has a dark or bright concave part, as characterized and oftenobserved by a bright field. Here, FIG. 7 is a graph showing a typicaldetection result when the alignment mark 110 shown in FIG. 6 isoptically detected.

The alignment signal processor 30 signal-processes an image of thealignment mark 110, which has been thus taken. The alignment signalprocessor 30 in the instant embodiment uses template matching tocalculate a position of the alignment mark 110. The template matchingcorrelates an obtained signal S shown in FIG. 8B with a model signal(template) T shown in FIG. 8A, to which the template has been stored inthe apparatus, and detects a position having the highest correlation asa center of the alignment mark. A resolution of {fraction (1/10)} to{fraction (1/50)} is available by calculating a barycenter pixelposition in an area that ranges several pixels to the right and leftfrom the peak pixel using a function of a correlation value E shown inFIG. 8C. Here, FIGS. 8A to 8C are views for explaining the templatematching applicable to the detection result shown in FIG. 7.

The template matching is expressed by the following Equation 1, where Sis a signal obtained by the sensor, T is a model signal, and E is acorrelation result: $\begin{matrix}{{E(X)} = \frac{1}{\sum\limits_{J = {- k}}^{k}\quad\left\lbrack {{S\left( {X + J} \right)} - {T(J)}} \right\rbrack^{2}}} & \left\lbrack {{EQUATION}\quad 1} \right\rbrack\end{matrix}$

FIG. 8 shows a relationship among the signal S, model signal T, andcorrelation values E. FIG. 8 shows processing to one mark-element imageamong four mark elements 112. As applied similarly hereinafter, thetemplate matching detects image positions on the sensor for other threemark elements.

The mark-element image positions X1(n), X2(n), X3(n) and X4(n) (withunit of pixel) are calculated by using the template matching. Here, “n”is a template number. The entire center coordinate Xa(n) of thealignment mark 110 is determined as expressed by Equation 2 by averagingcenter positions X1(n) to X4(n) of four mark element images:$\begin{matrix}{{{Xa}(n)} = \frac{{{X1}(n)} + {{X2}(n)} + {{X3}(n)} + {{X4}(n)}}{4}} & \left\lbrack {{EQUATION}\quad 2} \right\rbrack\end{matrix}$

Equation 3 calculates a positional offset Xw(n) of the alignment mark110 on the wafer 100, where Xa is the average value of the mark image onthe wafer 100 that has been obtained for each template, M is the imagingmagnification of the alignment optical system 200, and Px is the pixelpitch of the line sensor 226 in the alignment measuring direction:$\begin{matrix}{{{Xw}(n)} = \frac{{Xa}(n)}{{Px} \cdot M}} & \left\lbrack {{EQUATION}\quad 3} \right\rbrack\end{matrix}$

Based on Equation 3, the alignment signal processor 160 calculates apositional offset amount X1 of the alignment mark 110 from a best focusimage signal obtained by the line sensor 226, and a positional offsetamount X2 of the alignment mark 110 from the line sensor 226. Theposition X of the alignment mark 110 is determined by using thepositional offset amounts X1 and X2 as in the following processingmethod.

A description will now be given of an alignment method for the wafer 100based on measurement values of positions of the alignment mark 110. Theinstant embodiment adopts the advanced global alignment (“AGA”), whichselects some shots (these selected shots are referred to as “sampleshots”) among all the shots (or chips) on a wafer, and detects positionsof the alignment marks in the shots.

FIG. 9 shows an offsetting shot arrangement on the wafer 100 relative tothe XY coordinate system on the wafer stage 20 in the exposure apparatus1. An offset of the wafer 100 can be described with six parametersincluding a shift Sx in the direction x, a shift Sy in the direction y,an inclination θx to the x-axis, an inclination θy to the y-axis, amagnification Bx in the direction x, and a magnification By in thedirection y. Bx and By represent an expansion and contraction of thewafer 110 based on wafer stage feeding in the exposure apparatus 1, andare caused by applying heat to the wafer 100 as in a film formation andetching in the semiconductor process.

Equations 4 and 5 are defined as follows, where Ai is measurement valuesfor each measured AGA sample shot, Di is designed positional coordinatesof the alignment mark 110 in the sample shot, and “i” is a measurementshot number: $\begin{matrix}{{Ai} = \begin{bmatrix}{xi} \\{yi}\end{bmatrix}} & \left\lbrack {{EQUATION}\quad 4} \right\rbrack \\{{Di} = \begin{bmatrix}{Xi} \\{Yi}\end{bmatrix}} & \left\lbrack {{EQUATION}\quad 5} \right\rbrack\end{matrix}$

AGA conducts the primary coordinate conversion D′i expressed by Equation6 using the above six parameters (Sx, Sy, θx, θy, Bx, By) that representwafer's positional offsets: $\begin{matrix}{{D^{\prime}i} = {{\begin{pmatrix}{Bx} & {{- \theta}\quad y} \\{\theta\quad x} & {By}\end{pmatrix}{Di}} + \begin{pmatrix}{Sx} \\{Sy}\end{pmatrix}}} & \left\lbrack {{EQUATION}\quad 6} \right\rbrack\end{matrix}$

Equation 6 approximated θx and θy are minute (≈0) and Bx=By≈1, cos θ=1,sine θ=θ, θx·Bx=θx, θy·By=θy, etc.

FIG. 10 shows a primary coordinate conversion of Equation 6. Referringto FIG. 10, the alignment mark 110 is located at a position W on thewafer 100, offset by Ai from a position M as a designed position, andshows a positional offset (or residue) Ri after the coordinateconversion D′i:Ri=(Di+Ai)−Di′  [EQUATION 7]

AGA adopts the least square method to minimize the residue Ri for eachsample shot, or calculates (Sx, Sy, θx, θy, Bx, By) that minimizesaveraged square sum of the residue Ri using Equations 8 and 9 below:$\begin{matrix}{V = {{\frac{1}{n}{\sum{{Ri}}^{2}}} = {\frac{1}{n}\underset{i = 1}{\overset{t = n}{\sum\quad}}\quad{{\begin{pmatrix}{xi} \\{yi}\end{pmatrix} - {\begin{pmatrix}{{Bx} - 1} & {{- \theta}\quad y} \\{\theta\quad x} & {{By} - 1}\end{pmatrix}\begin{pmatrix}{Xi} \\{Yi}\end{pmatrix}} - \begin{pmatrix}{Sx} \\{Xy}\end{pmatrix}}}^{2}}}} & \left\lbrack {{EQUATION}\quad 8} \right\rbrack \\{\begin{pmatrix}{\delta\quad{V/\delta}\quad{Sx}} \\{\delta\quad{V/\delta}\quad{Sy}} \\{\delta\quad{V/\delta}\quad{Rx}} \\{\delta\quad{V/\delta}\quad{Ry}} \\{\delta\quad{V/\delta}\quad{Bx}} \\{\delta\quad{V/\delta}\quad{By}}\end{pmatrix} = 0} & \left\lbrack {{EQUATION}\quad 9} \right\rbrack\end{matrix}$

AGA parameters (Sx, Sy, θx, θy, Bx, By) are obtained by substituting forEquations 8 and 9, measurement values (xi, yi) for each sample shot andalignment-mark designed position (Xi, Yi). The exposure follows thealignment for each shot on the wafer 110 based on the calculatedparameters.

A description will be given of the wavelength selecting means 250 in thealignment optical system 200 shown in FIG. 5. FIG. 11 is a plane viewshowing one exemplary structure of the wavelength selecting means 250 inthe alignment optical system 200 shown in FIG. 5. Referring to FIG. 11,the wavelength selecting means 250 of the instant embodimentconcentrically arranges ten band-pass interference filters 254 a to 254j on a disc 252, and illuminates the alignment mark 110 withillumination light having an arbitrary wavelength that is determined byrotating the disc 252 and arranging arbitrary one of the band-passinterference filters 254 a to 254 j on an optical path of theillumination light. FIG. 12 is a spectral characteristic diagram of theband-pass interference filters 254 a to 254 j in the wavelengthselecting means 250 shown in FIG. 11. Referring to FIG. 12, each of theband-pass interference filters 254 a to 254 j in the instant embodimentis configured to have a transmitting wavelength width of about 30 nm sothat a transmitting center wavelength shifts by 20 nm.

The wavelength selecting means 250 can use a band-pass filter that ismanufactured, for example, by combining a birefringence plate and aferroelectric liquid crystal cell. Alternatively, the wavelengthselecting means 250 may use a Ti sapphire laser or a combination ofplural light sources having different wavelengths.

A description will now be given of an optimal wavelength selectingmethod that is applicable when the shape of the alignment mark 110 hasan asymmetric error. FIG. 13 is a sectional view of the alignment mark110. The uneven coating of the resist 120 occurs so that a concaveportion is formed at a position offset from the alignment mark 110 by Sfin the measuring direction. This embodiment uses the resist 120'scoating thicknesses Rt of 320 nm, 368 nm, 416 nm, and 464 nm, calculatesthe alignment signal based on the simulation result of the alignmentmark image for the wavelength of the illuminating alignment light, andobtains a position of the alignment mark 110 by using the above templatematching. The alignment error is a difference between an ideal positionof the alignment mark 110 on the wafer 100 where there is no asymmetriccomponent, and a position (or a detected value) of the alignment mark110 obtained from the alignment signal.

FIG. 14A is a view showing a relationship between the wavelength of thealignment light and the alignment error. In FIG. 14A, the abscissa axisis the wavelength of the alignment light, and the ordinate axis is thealignment error (an inclination of the alignment error relative to theshift amount of the resist 120 in FIG. 13). As shown in FIG. 14A, thereis an alignment light wavelength that provides alignment error of zeroaccording to the coating thickness of the resist 120. This means thateven when the resist 120 is unevenly coated, a proper selection of thewavelength of the alignment light that zeros the alignment error enablesthe position of the alignment mark 110 to be precisely detected on thewafer 100. Therefore, it is understood as discussed above that thewavelength selecting means 250 can preferably vary the wavelength of thealignment light at the pitches of 20 nm.

The instant inventors have addressed the resist 120 that is locatedoutside the mark elements 112 or a non-mark part B (see the area B inFIG. 13), in order to select a wavelength that zeros the alignmenterror. The non-mark part B, as used herein, is defined as an area thatis spaced from the edge of the mark element 112 by a predetermineddistance, has a constant coating thickness and coating structure, andmaintains the reflectance constant. When the alignment mark 110 includesplural mark elements 112 as in the instant embodiment, it is preferablethat the space between the mark elements 112 is set to the non-mark partB.

FIG. 14B is a spectral reflectance characteristic diagram at thenon-mark part B shown in FIG. 13. Referring to FIGS. 14A and 14B, thealignment error is the smallest in an area that provides little changesof the reflectance of the non-mark part B, or at the wavelength thatminimizes or maximizes the reflectance. This is because even when theresist 120 is applied asymmetrically with respect to both ends of theedge of the mark element 112 in the alignment mark due to a differenceof the coating thickness of the resist 120, the reflectance differencecaused by the coating thickness of the resist 120 is so small that thealignment signal maintains symmetry. Alternatively, this is because theasymmetric amount is small and the error is extremely small, and thus aposition of the alignment mark 110 is detectable. While the alignmenterror becomes minimum at the wavelengths that minimize and maximize thenon-mark part B, it is preferable to use the wavelength that maximizesthe reflectance of the non-mark part B since the contrast of thealignment signal lowers in the bright field illumination at thewavelength that minimizes the reflectance of the non-mark part B.

A description will now be given of a method for calculating a wavelengththat maximizes the reflectance at the non-mark part B. A first methodacquires the alignment signal and compares the light intensity at thenon-mark part B. The above AGA is conducted for plural wavelengths andthe wafer 100 after the resist 120 is applied to the wafer 100, on whichthe alignment mark 110 has been formed. Then, the alignment signal isacquired for each wavelength and sample shot. Next, Isin(λ) that is anaverage of alignment signal intensities at the non-mark parts B for allthe sample shots is calculated for each wavelength λ. In order tocorrect the incident light intensity of the alignment light for eachwavelength and the transmittance in the alignment optical system 200 foreach wavelength, this embodiment uses the alignment optical system 200to photograph a reference mark table that is installed on the waferstage 20 and has a known spectral reflectance Rref(λ) Similarly, thealignment signal's intensity Iref(λ) is obtained for each wavelength inadvance.

Thereby, the reflectance R(λ) at the non-mark part B can be calculatedby the following Equation 10:R(λ)=Isin(λ)×Rref(λ)/Iref(λ)  [EQUATION 10]

The instant embodiment calculates the reflectance R(λ) at the non-markpart B from Equation 10, selects a wavelength that maximizes thereflectance, and uses the alignment light that has this wavelength.

In order to calculate the reflectance at the non-mark part B withoutusing the line sensor 226 for generating the alignment signal, areflectance measuring unit 300 may be provided, as shown in FIG. 15,which partially divides the illumination light from the alignment lightsource 211 in the alignment optical system 200, illuminates only thenon-mark part B of the alignment mark 110, and measures the reflectedlight on a photodiode 310. The wafer 100 for obtaining the alignmentsignal may be exposed after the optimal wavelength is selected (whichmaximizes the reflectance of the non-mark part B), and the wafer 100 isaligned using the alignment light having this wavelength. Here, FIG. 15is a block diagram of one exemplary structure of the reflectancemeasuring unit 300.

A second method for calculating a wavelength that maximizes thereflectance at the non-mark part B calculates the reflectance from therefractive index and coating thickness of the material of the alignmentmark 110 (or the refractive index and coating thickness of the resist120). As shown in FIG. 16, the reflectance Ri is given by the followingEquation 11, where n1 is a refractive index of the resist 120, and n2 isa refractive index of the wafer 100. FIG. 16 is a view showing therefractive index and the coating thickness of the wafer 110 and theresist 120. $\begin{matrix}{{Ri} = {1 - \frac{4{n1}^{2}{n2}}{{{n1}^{2}\left( {1 + {n2}} \right)}^{2} + {\left( {{n2}^{2} - {n1}^{2}} \right)\left( {1 - {n1}^{2}} \right)\sin^{2}\delta\quad i}}}} & \left\lbrack {{EQUATION}\quad 11} \right\rbrack\end{matrix}$δi is a phase difference between the reflected light from a frontsurface 120 a of the resist 120 and the reflected light from a backsurface 120 b of the resist 120, and corresponds to a difference betweentheir optical-path lengths. δi is given by Equation 12 below, where d1is a coating thickness of the resist 120, and θi is an incident anglefrom the resist 120 to the wafer 100: $\begin{matrix}{{\delta\quad i} = {\frac{2\pi}{\lambda}{n1d1}\quad\cos\quad\theta\quad i}} & \left\lbrack {{EQUATION}\quad 12} \right\rbrack\end{matrix}$

When the illumination light is perpendicularly incident upon the wafer100, the wavelength λ that maximizes the reflectance is given byEquations 13 and 14 below:When n1<n2, n 1 d 1=λ/2+mλ/2 (m=0,1,2, . . . ).  [EQUATION 13]When n1>n2, n 1 d 1=λ/4+mλ/2 (m=0,1,2, . . . ).  [EQUATION 14]

More specifically, the actual alignment optical system 200 often usespartial coherent illumination, and it is preferable to consider theoblique incident angle for the coherency a in addition to theperpendicular illumination for the calculation.

Since an optical-path length between the reflected light from the frontsurface 120 a of the resist 120 and the reflected light from the backsurface 120 b of the resist 120 becomes longer in the oblique incidentlight than that in the perpendicular incident light, the wavelength thatmaximizes the reflectance shifts farther by about several nanometerstowards the short wavelength side than that of the perpendicularincidence. A stricter calculation finds, as shown in FIG. 16, thereflectance Ri based on Equation 11 for each incident angle, where J isthe number of incident rays corresponding to the coherency σ, and i isan incident ray number corresponding to the incident angle, then findsthe reflectance R as an average value as expressed by Equation 15, andfinally finds a wavelength that maximizes the reflectance:$\begin{matrix}{R = {\frac{1}{Ji}{\sum\limits_{i = 1}^{J}\quad{Ri}}}} & \left\lbrack {{EQUATION}\quad 15} \right\rbrack\end{matrix}$

The alignment may select one of the band-pass filters 254 a to 254 j inthe wavelength selecting means 250 in the alignment optical system 200,which one has the closest central wavelength to the wavelength thatmaximizes the reflectance calculated from Equations 13, 14 or 15. Thealignment may use the alignment light that has a wavelength in apredetermined range centering on the wavelength that maximizes thereflectance. The predetermined range is 30 nm centering on thewavelength that maximizes the reflectance. While the instant embodimentdescribes a calculation of the reflectance of a two-layer structure(i.e., the wafer 100 and the resist 120), the present embodiment isapplicable to the alignment mark 110 that includes a multilayer coating,by using the known equation of the multilayer coating reflectance.

Alternatively, the exposure apparatus 1 may include an input part usedby a user to input an optical constant and coating thickness of amaterial of the alignment mark 110 (resist 120), and a calculation unitthat automatically calculates a wavelength that maximizes thereflectance at the non-mark part by using the input optical constant andcoating thickness. The alignment light having a wavelength that is leastaffected by the asymmetry of the alignment mark 110 may be automaticallyselected and used for the alignment. An alternative embodimentpreviously stores the alignment light's wavelength that maximizes thereflectance at the non-mark part for each optical constant and coatingthickness of the resist 120, acquires information of the resist 120, andselects the wavelength of the alignment light which maximizes thereflectance at the non-mark part from the database.

The instant embodiment applies the template matching as a signalprocessing method for detecting a position of the alignment mark 110from the alignment signal, but may use another signal processing method,such as a return symmetric processing. The inventive wavelengthselecting method can reduce alignment signal errors due to the asymmetryof the alignment mark 110, and achieve highly precise alignment.

While the instant embodiment ascribes the asymmetry of the alignmentmark 110 to the uneven coating of the resist 120, the asymmetry of thealignment mark 110 is not exclusively caused by the resist 120 and canbe caused by the asymmetric coating thickness of the opaque coatingformed on the alignment mark 110.

FIG. 17 shows changes of the reflectance at the non-mark part for eachwavelength when the alignment light that provides the highest contrastof the alignment signal is used but the alignment error cannot beeliminated. It is understood from FIG. 17 that the reflectance at thenon-mark part becomes maximum when the wavelength of the alignment lightis 580 nm, and corresponds to the wavelength that zeros the alignmenterror. In other words, the inventive wavelength selecting method canmore effectively reduce the alignment errors than the selection of thewavelength of the alignment light based on the alignment signal'scontrast. Here, FIG. 17 is a view showing a relationship between thereflectance to the alignment light's wavelength, the alignment signal'scontrast, and the alignment error.

While the illumination optical system 210 in the alignment opticalsystem 200 time-sequentially changes plural alignment lights havingdifferent wavelengths in the above embodiment and illuminates thealignment mark 110, the following embodiment uses an alignment opticalsystem 200A that illuminates the alignment mark 110 using pluralalignment lights having different wavelengths at the side of theillumination optical system 210 and divides the lights into signalshaving different wavelengths at the side of the imaging optical system220.

FIG. 18 is a block diagram of principal components of the alignmentoptical system 200A. Referring to FIG. 18, the lens 212 enlarges andcollimates illumination light from the alignment light source 211, andthe lens 213 condenses the resultant light again. During this period,the variable aperture stop 214 adjusts the coherency (a) of theillumination light. The aperture 215 is located at a position conjugatewith the wafer 100, and serves as a field stop for preventingunnecessary light from illuminating an area around the alignment mark110 on the wafer 100.

The light condensed by the lens 213 is collimated by the lens 216,reflected by the beam splitter 221, passes through the lens 222, andilluminates the alignment mark 110 on the wafer 100. The light reflectedfrom the alignment mark 110 passes through the lens 222, beam splitter221, lenses 223, 224 and 225, and is received by the line sensor 226.

Dichroic mirrors 228 and 229 are provided along the optical path of theimaging optical system 220. The light reflected from the dichroic mirror228 is received by a line sensor 226A via a lens 225A, and the lightreflected from the dichroic mirror 229 is received by a line sensor 226Bvia a lens 225B.

The designed spectral reflectance characteristic of the dichroic mirror228 is as shown by RL in FIG. 19, and effectively reflects the lighthaving a center wavelength of λc1=0.5 μm and a wavelength width 50 nm,and transmits the light at the longer wavelength side. Therefore, thealignment signal generated by the line sensor 226A is formed by thelight having a center wavelength of λc1=0.5 μm and a wavelength width 50nm.

The designed spectral reflectance characteristic of the dichroic mirror229 is as shown by ML in FIG. 19, and effectively reflects the lighthaving a center wavelength of λc3=0.61 μm and a wavelength width 50 nm,and transmits the light at the longer wavelength side. Therefore, thealignment signal generated by the line sensor 226B is formed by thelight having a center wavelength of λc1=0.61 μm and a wavelength width50 nm.

The light that passes the dichroic mirrors 228 and 229 and is receivedby the line sensor 226 is the light having a center wavelength ofλc3=0.61 μm and a wavelength width 50 nm as shown by TL in FIG. 19.Here, FIG. 19 is spectral reflectance characteristics of the dichroicmirrors 228 and 229 shown in FIG. 18.

Thus, the alignment optical system 200A uses three line sensors 226,226A and 226B and simultaneously forms the alignment signals havingdifferent wavelengths of 0.50 μm, 0.555 μm, and 0.61 μm. The alignmentoptical system 200A has a more complicated structure of the imagingoptical system 220 than the alignment optical system 200, but has a moreadvantageous throughput since the alignment optical system 200A cansimultaneously generate the alignment signals having differentwavelengths. Moreover, this embodiment uses the dichroic mirror torestrict the wavelength width, and arranges, on the optical path afterthe light separation, the chromatic aberration correcting means thatcorresponds to only the limited wavelength width, facilitatingcorrections to the chromatic aberration. While the instant embodimentuses the lights having three different wavelengths, the increased numberof dichroic mirrors would provide more alignment signals having morewavelengths at fine wavelength pitches.

FIG. 20 is a block diagram of principal components in the alignmentoptical system 200B as a variation of the alignment optical system 200Ashown in FIG. 18. The alignment optical system 200B characteristicallyuses a two-dimensional area sensor for the sensor 230, and divides thelight into a direction perpendicular to the measuring direction of thealignment mark 110.

Referring to FIG. 20, the lens 212 enlarges and collimates illuminationlight from the alignment light source 211A for emitting plural lightshaving plural wavelengths, and the lens 213 condenses the resultantlight again. During this period, the variable aperture stop 214 adjuststhe coherency (σ) of the illumination light. The aperture 215 is locatedat a position conjugate with the wafer 100, and serves as a field stopfor preventing unnecessary light from illuminating an area around thealignment mark 110 on the wafer 100.

The light condensed by the lens 213 is collimated by the lens 216,reflected by the beam splitter 221, passes through the lens 222, andilluminates the alignment mark 110 on the wafer 100. The light reflectedfrom the alignment mark 110 passes through the lens 222, beam splitter221, lenses 223, 224 and 225, and is received by the area sensor 230.

A band-pass filter 231 is arranged, as shown in FIG. 21, at a positionconjugate with the alignment mark 110 along the optical path of theimaging optical system 220. The band-pass filter 231 forms three typesof band-shaped multilayer coatings 231 a to 231 c on a transparentplate, as shown in FIG. 21, in the direction perpendicular to themeasuring direction of the alignment mark 110. The instant embodimentdesigns the transmission bands of the multilayer coatings 231 a to 231 cof 480 nm to 520 nm, 520 nm to 560 nm, and 560 nm to 600 nm,respectively. The band-pass filter 231 is located at a positionconjugate with the alignment mark 110, and an image 110 z of thealignment mark 110 is formed on the band-pass filter 231. Here, FIG. 21is a sectional view of the band-pass filter 231 of the alignment opticalsystem 200B shown in FIG. 20.

Since the area sensor 230 is also located at a position conjugate withthe alignment mark 110 and the band-pass filter 231, the image 110 z ofthe alignment mark 110 and the band-pass filter 231 are formed on thearea sensor 230 again, as shown in FIG. 22. In other words, on the areasensor 230, the image 110 a of the alignment mark 110 can be obtainedwhen the alignment mark 110 is illuminated by the alignment light havinga different wavelength in a direction perpendicular to the measuringdirection of the alignment mark 110. Therefore, an alignment signalhaving a desired wavelength can be obtained, as shown in FIG. 22, byselecting a signal line of the area sensor 230. In other words, thisembodiment selects a signal line corresponding to a wavelength within apredetermined range centering on the wavelength that maximizes thereflectance at the non-mark part of the alignment mark 110, andsignal-processes the alignment signal generated from the signal line,and detects a position of the alignment mark 110 with high precision.Here, FIG. 22 is a plane view showing the image 110 z of an alignmentmark 110 on the area sensor 230 in the alignment optical system 200Bshown in FIG. 20.

While the alignment optical system 200B uses the lights having threedifferent wavelengths, an alternative embodiment can obtain morealignment signals having more wavelengths at finer wavelength pitches byincreasing the number of multilayer coatings in the band-pass filter231. The band-pass filter 231 can be arranged at the side of theillumination optical system 210. Since FIG. 20 arranges the band-passfilter 231 at a position of the aperture 215 conjugate with thealignment mark 110, the band-pass filter 231 may be provided at thetransmission part (or opening part) of the aperture 215 so that theband-pass filter 231 serves as the field stop.

FIG. 23 is a block diagram of principal components in an alignmentoptical system 200C as a variation of the alignment optical system 200Bshown in FIG. 20. While the alignment optical system 200C uses thetwo-dimensional area sensor for the sensor 230, similar to the alignmentoptical system 200B, the alignment optical system 200Ccharacteristically uses a diffraction grating in a directionperpendicular to the measuring direction of the alignment mark 110.

Referring to FIG. 23, the lens 212 enlarges and collimates illuminationlight from the alignment light source 211A for emitting plural lightshaving plural wavelengths, and the lens 213 condenses the resultantlight again. During this period, the variable aperture stop 214 adjuststhe coherency (σ) of the illumination light. The aperture 215 is locatedat a position conjugate with the wafer 100, and serves as a field stopfor preventing unnecessary light from illuminating an area around thealignment mark 110 on the wafer 100.

The light condensed by the lens 213 is collimated by the lens 216,reflected by the beam splatter 221, passes through the lens 222, andilluminates the alignment mark 110 on the wafer 100. The light reflectedfrom the alignment mark 110 passes through the lens 222, beam splitter221, lenses 223, 224 and 225, and is received by the area sensor 230.

A diffraction grating 240 is provided between the lens 225 and the areasensor 230. The diffraction grating 240 arranges a grating in adirection perpendicular to the measuring direction of the alignment mark110, and the light diffracted by the diffraction grating 240 is receivedby the area sensor 230 as shown in FIG. 24. Due to the spectraloperation of the diffraction grating 240, the alignment mark 110 isimaged on the area sensor 230 by the lights having continuouslydifferent wavelengths in a direction perpendicular to the measuringdirection of the alignment mark 110. Thereby, the alignment signalhaving an arbitrary center wavelength and wavelength width can begenerated by arbitrarily selecting and averaging signal lines of thearea sensor 230 in the direction perpendicular to the measuringdirection of the alignment mark 110. Here, FIG. 24 is a sectional viewshowing a spectral operation of the diffraction grating 240 in thealignment optical system 230C shown in FIG. 23.

This embodiment can detect the position of the alignment mark 110 byselecting a signal line corresponding to the wavelength within apredetermined range centering on the wavelength that maximizes thereflectance of the non-mark part of the alignment mark 110, andsignal-processing the alignment signal generated from the signal line.Since the alignment optical system 230C can form an image of thealignment mark corresponding to a wavelength that continuously changesin a direction perpendicular to the measuring direction of the alignmentmark 110, the number of signal lines of the area sensors 230 can bearbitrarily selected with the high degree of freedom of selecting thecentral wavelength and wavelength width.

In order to implement the alignment, the alignment optical systems 200,200A to 200C reduce the influence of the asymmetry of the alignment markdue to the semiconductor process, and improve the alignment accuracy andyield (or throughput) in the semiconductor device manufacturing process.

Turning back to FIG. 4, the controller 40 includes a CPU and a memory(not shown), and controls actions of the exposure apparatus 1. Thecontroller 40 is electrically connected to the illumination apparatus(not shown), the reticle stage (not shown), and the wafer stage 20. Thecontroller 40 positions the wafer 100 through the wafer stage 20 basedon the positional information of the alignment mark 110 from thealignment signal processor 30. The CPU cover-s any processorsirrespective of its name, such as an MPU, and controls each component'soperation. The memory includes a ROM and PAM, and stores a firmware foroperating the exposure apparatus 1.

In exposure, light emitted from an illumination apparatus (not shown),for example, Koehler-illuminates the reticle RC. The light that haspassed the reticle RC and reflects the reticle pattern forms an image onthe wafer 100 through the projection optical system 10. Since thealignment optical system 200 and alignment signal processor 30, etc.provide the exposure apparatus 1 with a highly precise alignment, theexposure apparatus 1 can provide excellent devices (such assemiconductor devices, LCD devices, image pick-up devices (such asCCDs), and thin film magnetic heads) with high throughput and economicalefficiency.

Referring now to FIGS. 25 and 26, a description will be given of anembodiment of a device manufacturing method using the above exposureapparatus 1. FIG. 25 is a flowchart for explaining the fabrication ofdevices (i.e., semiconductor chips such as IC and LSI, LCDs, CCDs,etc.). Here, a description will be given of the fabrication of asemiconductor chip as an example. Step 1 (circuit design) designs asemiconductor device circuit. Step 2 (mask fabrication) forms a maskhaving a designed circuit pattern. Step 3 (wafer preparation)manufactures a wafer using materials such as silicon. Step 4 (waferprocess), which is referred to as a pretreatment, forms actual circuitryon the wafer through photolithography using the mask and wafer. Step 5(assembly), which is also referred to as a posttreatment, forms into asemiconductor chip the wafer formed in Step 4 and includes an assemblystep (e.g., dicing, bonding), a packaging step (chip sealing), and thelike. Step 6 (inspection) performs various tests for the semiconductordevice made in Step 5, such as a validity test and a durability test.Through these steps, a semiconductor device is finished and shipped(Step 7).

FIG. 26 is a detailed flowchart of the wafer process in Step 4. Step 11(oxidation) oxidizes the wafer's surface. Step 12 (CVD) forms aninsulating film on the wafer's surface. Step 13 (electrode formation)forms electrodes on the wafer by vapor disposition and the like. Step 14(ion implantation) implants ions into the wafer. Step 15 (resistprocess) applies a photosensitive material onto the wafer. Step 16(exposure) uses the exposure apparatus 1 to expose the circuit patternon the mask onto the wafer. Step 17 (development) develops the exposedwafer. Step 18 (etching) etches parts other than the developed resistimage. Step 19 (resist stripping) removes disused resist after etching.These steps are repeated, and multilayer circuit patterns are formed onthe wafer. The device fabrication method of this embodiment maymanufacture higher quality devices than the conventional one. Thus, thedevice fabrication method using the exposure apparatus 1 and theresultant devices constitute one aspect of the present invention.

Referring now to the accompanying drawings, a description will be givenof the exposure apparatus 301 according to another embodiment of thepresent invention. While the previous embodiment is directed to awavelength selection in detecting a position of an alignment mark on awafer (or a position in a direction on the wafer plane), this embodimentis directed to a wavelength selection in the water's focus detection ora wafer's position in an optical-axis direction of the projectionoptical system. FIG. 27 is a schematic block diagram of a structure ofthe exposure apparatus 301 according to one aspect of the presentinvention.

The exposure apparatus 301 is a projection exposure apparatus thatexposes onto a wafer the circuit pattern on a reticle 320 in astep-and-scan manner. Such an exposure apparatus is suitable for asub-micron or quarter-micron lithography process. The exposure apparatus301 includes, as shown in FIG. 27, an illumination apparatus 310, areticle stage 325 mounted with the reticle 320, a projection opticalsystem 330, a wafer stage 345 mounted with a wafer 340, a focus/tiltdetecting system 350, and a controller 360. The controller 360 includesa CPU and a memory, is electrically connected to the illuminationapparatus 310, the reticle stage 325, the wafer stage 345, and thefocus/tilt detecting system 350, and controls the operations of theexposure apparatus 301. The controller 360 provides operations andcontrols for optimally setting the wavelength of the light used by thefocus/tilt detecting system 350, which will be described later, todetect the surface position of the wafer 340 in the instant embodiment.

The illumination apparatus 310 illuminates the reticle 320, on which acircuit pattern to be transferred is formed, and includes a light sourceunit 312 and an illumination optical system 314.

The light source unit 312 uses as a light source, such as an ArF excimerlaser with a wavelength of approximately 193 nm and a KrF excimer laserwith a wavelength of approximately 248 nm, but a type of laser is notlimited to excimer laser and a F₂ laser with a wavelength ofapproximately 153 nm and the extreme ultraviolet (“EUV”) light having awavelength of 20 nm or below can be used.

The illumination optical system 314 is an optical system thatilluminates a target surface using the light emitted from the lightsource unit 312. The illumination optical system 314 in the instantembodiment converts the light into a predetermined form of exposure slitoptimal to the exposure, and illuminates the reticle 320. Theillumination optical system 314 includes a lens, a mirror, a lightintegrator, a stop, and the like, for example, a condenser lens, afly-eye lens, an aperture stop, a condenser lens, a slit, and an imagingoptical system in this order. The illumination optical system 314 canuse any light whether it is axial or non-axial light. The lightintegrator may include a fly-eye lens or an integrator formed bystacking two sets of cylindrical lens array plates (or lenticularlenses), and be replaced with an optical rod or a diffractive element.

The reticle 320 is made, for example, of quartz, has a circuit patternto be transferred, and is supported and driven by the maskstage 325.Diffracted light emitted from the reticle 320 passes the projectionoptical system 330, thus and then is projected onto the wafer 340. Thereticle 320 and the wafer 340 are located in an optically conjugaterelationship. The reticle 320 and the wafer 340 are scanned at the speedratio of the reduction ratio, thus transferring the pattern on thereticle 320 to the wafer 340. The exposure apparatus 301 includes anoblique incident system of a reticle detecting means 370 for detecting aposition of the reticle 320, and the reticle 320 is positioned in place.

The reticle stage 325 supports the reticle 320 via the reticle chuck(not shown), and is connected to a moving mechanism (not shown). Themoving mechanism (not shown) includes a linear motor, etc., and can movethe reticle 320 by driving the reticle stage 325 in the XYZ axesdirections and rotating direction around each axis.

The projection optical system 330 serves to image the light from anobject surface onto an image surface, and the diffracted light from thepattern on the reticle 320 onto the wafer 340 in this embodiment. Theprojection optical system 330 may use an optical system solely includinga plurality of lens elements, an optical system including a plurality oflens elements and at least one concave mirror (a catadioptric opticalsystem), an optical system including a plurality of lens elements and atleast one diffractive optical element such as a kinoform, and a fullmirror type optical system, and so on. Any necessary correction of thechromatic aberration may use a plurality of lens units made from glassmaterials having different dispersion values (Abbe values), or arrange adiffractive optical element such that it disperses in a directionopposite to that of the lens unit.

The wafer 340 is an object to be exposed such as a wafer and a liquidcrystal plate, and photoresist is applied onto it. The wafer 340 is alsoan object whose position is to be detected by the focus/tilt detectingsystem 350. The wafer 340 in another embodiment is replaced with aliquid crystal substrate or another object to be exposed.

The wafer stage 345 supports the wafer 340 via a wafer chuck (notshown). Similar to the reticle stage 325, the wafer stage 345 is movedby a linear motor in the XYZ axes directions and rotating directionaround each axis. The positions of the reticle stage 325 and the waferstage 345 are monitored, for example, by a laser interferometer and thelike, so that both are driven at a constant speed ratio. The wafer stage345 is installed on a stage stool supported on the floor and the like,for example, via a damper, and the reticle stage 325 and the projectionoptical system 330 are installed on a lens barrel stool (not shown)supported, for example, via a damper to the base frame placed on thefloor.

The focus/tilt detecting system 350 in the instant embodiment obtainspositional information of the surface position of the wafer 340 in theZ-axis direction during exposure. The focus/tilt detecting system 350introduces lights to plural measuring points to be measured on the wafer340, and then guide the lights to individual sensors, and detect thetilt of the exposure surface from the positional information of thedifferent positions (or the detection result).

The focus/tilt detecting system 350 includes, as shown in FIG. 28, anillumination unit 352 for introducing the light to the surface of thewafer 340 at a high incident angle, a detector 354 for detecting animage offset of the reflected light that is reflected from the surfaceof the wafer 340, and an operation part 356. The illumination unit 352includes a light source 354A, a light synthesizer means 352B, a patternplate 352C, an imaging lens 352D, and a mirror 352E. The detector 354includes a mirror 354A, a lens 354B, an optical demultiplexer means354C, and a light receiving unit 354D. Here, FIG. 28 is an enlargedblock diagram showing a structure of the focus/tilt detecting system350. FIG. 28 omits from the illumination part 352 a lens unit necessaryto illuminate the pattern plate 352C at the uniform light intensitydistribution, and a lens unit for correcting the chromatic aberration.

Referring to FIG. 28, the lights having wavelengths λ1, λ2 and λ3emitted from the light sources 352Aa, 352Ab and 352Ac, such as an LEDand a halogen lamp, pass the light synthesizer means 352B that includesa mirror 352Ba and dichroic mirrors 352 Bb and 352Bc, and illuminatesthe pattern plate 352C, on which a pattern, such as a slit, is formed.The light that passes the pattern plate 315 images on the wafer 340 viathe imaging lenses 352D and 352E.

The light reflected on the wafer 340 is received by the light receivingunit 354D that includes light receiving elements 354Da, 354Db and 354Dc,such as a CCD device and a line sensor. The optical demultiplexer means354C is provided before the light receiving unit 354D, and includes themirror 354Ca and dichroic mirrors 354Cb and 354Cc. The lightdemultiplexer means 354C serves to divide the lights having differentwavelengths so that the light receiving element 354Da receives the lightfrom the light source 352Aa, the light receiving element 354Db receivesthe light from the light source 352Ab, and the light receiving element354Dc receives the light from the light source 352Ac. The pattern imageon the pattern plate 352C of the wafer 340's surface re-images on thelight receiving elements 354Da to 354Dc by the lens 354B.

As the wafer 340 moves in the longitudinal direction (or the Z-axisdirection) via the wafer stage 345, the pattern image of the patternplate 352C moves in the lateral direction (or the X-axis direction) onthe light receiving unit 354D. Therefore, the focus tilt/detectingsystem 350 detects the surface position of the wafer 340 for eachmeasuring point by using the operation part 356 to calculate theposition of the pattern image.

A description will now be given of a pattern on the pattern plate 352Cand the signal waveform detected by the detector 354. FIG. 29 is a planeview of one example of the pattern plate 352C. Referring to FIG. 29, thepattern plate 352C has a grating pattern 352Cb that arranges fourrectangular shielding patterns 352Cc on the transmission area 352Ca.FIG. 30 shows a waveform detected by the detector 354 when the gratingpattern 352Cb of the pattern plate 352C is used. When the lightreceiving elements 354Da to 354Dc use a two-dimensional sensor, a signalwaveform is obtained by integrating (or averaging) the light intensityin the direction perpendicular to the arranging direction of theshielding patterns 352Cc. When the light receiving elements 354Da to354Dc use a one-dimensional line sensor, it is advantageous to use acylindrical lens having a power in the direction perpendicular to thearranging direction of the shielding patterns 352Cc to obtain the signalwaveform through the optical integration because S/N (i.e., a signal tonoise ratio) of the signal waveform improves.

FIG. 30A shows an example of a signal waveform obtained from the lighthaving the wavelength λ1. FIG. 30B shows an example of a signal waveformobtained from the light having the wavelength λ3. The signal waveformshown in FIG. 30A has a good symmetry of the image of the gratingpattern 352Cb, whereas the waveform signal shown in FIG. 30B has apartial asymmetry. Therefore, it is understood that the waveform signaleven at the same measuring point has dependency upon the light'swavelength.

Referring to FIGS. 31 to 33, a description will be given of thedependency of the waveform signal detected by the detector 354 relativeto the illumination light's wavelength. FIG. 31 is a view for explaininga difference of the reflectance caused by the uneven coating thicknessof the resist 342 and the pattern step 341 on the wafer 340. Thereflectance of the wafer 340 to which the resist 342 is applied isdetermined by the interference between the reflected light from theresist front surface 342 a and the reflected light from the resist backsurface 342 b (or an interface between the wafer 3.40 and the resist342). On the wafer 340, a coating thickness Rt′ of the resist 342 in anarea E2 having a pattern step 341 is larger than a coating thickness Rtof the resist 342 in an area E1 having no pattern step 341. Therefore,an optical-path length dA between the reflected light ka1 of the lightirradiated onto the area E1 at the resist front surface 342 a andreflected light ka2 of the light irradiated onto the area E1 at theresist back surface 342 b is different from an optical-path length dBbetween the reflected light kb1 of the light irradiated onto the area E2at the resist front surface 342 a and reflected light kb2 of the lightirradiated onto the area E2 at the resist back surface 342 b. Therefore,the areas E1 and E2 have a difference in reflectance. When the light isirradiated onto the areas having differences in reflectance, theasymmetric signal waveform occurs as shown in FIG. 30B.

Causes of the difference of reflectance that occurs on the wafer 340 arenot limited simply to the non-uniformity of the coating thickness of theresist, as shown in FIG. 31, but cover other reasons. FIG. 32 is a viewfor explaining a difference of the reflectance between an area E3 havinga small density of the step pattern 341 (or no step pattern 341) on thewafer 341 and an area E4 having a large density of the step pattern 341.

Referring to FIG. 32, the resist 342 has the same coating thicknessbetween the areas E3 and E4, and the reflected lights kc1 and kd1 haveapproximately the same reflectance on the resist front surface 342 a.However, the areas E3 and E4 have different densities of the steppattern 341 on the wafer 340, and the reflected light kc2 and kd2 havedifferent reflectances at the resist back surface 342 b (or theinterface between the wafer 340 and the resist 342). Moreover, when thestep pattern 341 on the wafer 340 is smaller than a wavelength of theillumination light, a phase jump due to a reflection called a structuralbirefringence, and a difference of reflectance occurs because a phasedifference occurs between the reflected lights kc2 and kd2 at the resistback surface 342 b.

In this way, the difference of the reflectance occurs due to the steppattern 341 (or the coating thickness of the resist 342) on the wafer340, but the difference of the reflectance occurs due to a difference ofa wavelength of the illumination light.

FIG. 33 is a graph showing a wavelength dependency of a reflectance ofthe wafer 340 relative to the resist 342's coating thickness. Supposethat the resist 342's coating thickness changes from Rt1 to Rt1±dR. Thereflectance varies only by d1 for the wavelength λ1 of the illuminationlight, whereas the reflectance varies by d3 for the wavelength λ3 of theillumination light. Even when the resist 342's coating thickness equallyvaries, the changing amount of the signal waveform for the reflectanceof the wavelength λ3 and its measuring precision improves, since thechanging amount of the reflectance of the wavelength λ1 is smaller thanthat of the reflectance of the wavelength λ3. In other words, themeasuring accuracy of the focus/tilt detecting system 350 variesaccording to a wavelength of the illumination light.

A description will be given of the measuring points of the surfaceposition (focus) of the wafer 340. This embodiment provides totallytwenty-one measuring points KP, i.e., seven points in the scan directionand three points in a direction perpendicular to the scan direction (orslit longitudinal direction) for an exposure area EE1 for one shot asshown in FIG. 34. This embodiment arranges three focus/tilt detectingsystems 350 in the slit longitudinal direction (or X-axis direction) formeasurements of three measuring points KP in the slit longitudinaldirection, and scans the wafer stage 345 in the X-axis direction formeasurements of seven measuring points in the scan direction (or Y-axisdirection). FIG. 34 is a plane view of one example of an arrangement ofthe measuring points KP on the wafer 340.

As shown in FIG. 35, since plural shots are exposed on the wafer 340,the wafer stage 345 is stepped and scanned for each shot and twenty-onemeasuring points KP shown in FIG. 34 are measured. Two or more measuringpoints KP are needed, because it is essential for the measuring pointsKP in the slit longitudinal direction to calculate the tilt amount ωy ofthe wafer 340 in the slit longitudinal direction as shown in FIG. 36.Moreover, when the tilt amount ωx of the wafer 340 in the scan directionin the slit, the measuring points KP that are differently located in thescan direction in the slit and a corresponding focus/tilt detectingsystem 350 should be provided as shown in FIG. 37. Here, FIG. 35 is aplane view showing a shot layout on the wafer 340. FIGS. 36 and 37 areplane views showing one exemplary arrangement of measuring points inslits of the wafer 340.

A description will be given of a method for selecting an optimalwavelength of light that illuminates the measuring points KP by thefocus/tilt detecting system 350. FIG. 38 is a flowchart for explainingan optimal wavelength selecting method of light irradiated by thefocus/tilt detecting system 350.

Referring to FIG. 38, surface positions of all the fourteen sample shotsare measured with lights having all the different wavelengths possessedby the focus/tilt detecting system in the shot layout on the wafer 340shown in FIG. 35 (step S602). Next, positions x1, x2, x3 and x4 (seeFIG. 30A) of images of the four shielding patterns 352Cc on the patternplate 352 are calculated based on the signal waveform measured by thefocus/tilt detecting system 350 (step S604), and grating intervals L1,L2 and L3 (see FIG. 30A) of the shielding pattern 352Cc are calculated(step S606). The grating intervals Lseij(λ) are calculated for all thewavelengths of lights and all the measuring points KP. Here, the suffix“s” is a sample shot number and S=1 to 14. The suffix “e” is an intervalnumber and e=1 to 3. The suffix “i” is an in-shot measuring positionnumber in a slit longitudinal direction, and i=1 to 3. The suffix “j” isan in-shot measuring position number in a scan direction, and j=1 to 7.λ is a wavelength of the illumination light, and λ=λ1 to λ3.

Next, the standard deviation σLij(λ) of the grating intervals atmeasuring positions (i, j) is calculated based on the grating intervalsLseij(λ) (step S608). The wavelength λ that minimizes the calculatedstandard deviation σLij(λ) is calculated, and this wavelength λ isdefined as the optimal wavelength λopt(i, j) of each measuring position(i, j) in the shot (step S610).

The reason why the wavelength λ that minimizes the calculated standarddeviation σLij(λ) is defined as the optimal wavelength λopt(i, j) ofeach measuring position (i, j) in the shot is that the coating thicknessof the resist 342 changes among shots on the wafer 340 due to the steppattern 341 and the resist 342's uneven application, and thus themeasurement precision improves as the reproducibility of the gratingintervals Lseij(λ) among shots improves.

After the optimal wavelength % opt(i, j) is selected, an average valueXa(i, j) of the positions x1 to x4 of the images of four shieldingpatterns 352Cc are calculated from the signal waveforms of the patternplate 352C illuminated by the optimal wavelength λopt(i, j) and thesurface position of the wafer 340 are measured.

The number of sample shots used to select the optimal wavelength is notlimited to fourteen, and all the shots on the wafer may be used formeasurements. The number of wafer to be measured is not limited to one;the optimization precision of the wavelength improves as several wafersare measured and the denominator of the grating intervals Lseij (λ)increases. While the instant embodiment forms the pattern plate 352C sothat the grating pattern 352Cb does not transmit the light, the entiresurface of the pattern plate 352C is made as a shielding part and onlythe grating pattern 352C may transmit the light.

A description will now be given of the overview of the surface positioncorrection of the wafer 340 by the focus and tilt measurements duringthe scan exposure. As shown in FIG. 39, before the conveoconcave wafer340 reaches the exposure position EP in the scan direction, the focusesand the tilts (referred to as “tilt X” hereinafter) in the exposure slitarea longitudinal direction (or a direction perpendicular to the scandirection) of the surface positions of the wafer 340 plural measuringpositions KP (or measuring position FP) that area arranged so that thesemeasuring points form a plane ahead of the exposure slits the waferstage 345 is driven to the exposure position EP for corrective drivingbased on the positional information detected by the focus/tilt detectingsystem 350. For example, in FIG. 28, a measurement of the surfaceposition of j=7 is measured while a position j=6 on the wafer 340 isbeing exposed. After the exposure to the position 6 ends, the positionof j=7 is exposed based on the measurement result of the surfaceposition of j=7, with corrective driving of the focus and tilt X. Here,FIG. 39 is a schematic perspective view showing an exposure area EP andthe focus and tilt measuring positions FP on the wafer 340.

FIG. 40 is a flowchart for explaining an exposure method using theexposure apparatus 301. Referring to FIG. 40, the wafer 340 supplied tothe exposure apparatus 301 (step S1002), and the focus/tilt detectingsystem 350 determines whether the wavelength of the light forilluminating the measuring positions KP should be optimized (stepS1004). Whether the wavelength is optimized is previously registered inthe exposure apparatus 301.

If the optimization of the wavelength is necessary, the surfacepositions are measured with respect to the sample shots shown in FIG. 35for all the wavelengths of the lights which the focus/tilt detectingsystem 350 can use (step S1006), and the surface position information isstored (step S1008). Whether there is an unmeasured sample shot is thendetermined (step S1010). If there is, measurements of surface positionsof the unmeasured sample shots and the storage of the surface positioninformation continue until there is no unmeasured sample shot.

When the measurements of surface positions of the unmeasured sampleshots and the storage of the surface position information end (or whenthere is no unmeasured sample shot), the optimal wavelength λopt(i, j)is calculated for each measuring point KP in the shot based on thesurface position information of each sample shot stored in the stepS1008 (step S1012).

On the other hand, when the optimization of the wavelength isunnecessary, it is determined whether the optimal wavelength of thelight used by the focus/tilt detecting system 350 to illuminate themeasuring point KP is calculated (step S1004). For example, whether theoptimal wavelength λopt(i, j) is calculated is determined based on theprior wafer. When the optimal wavelength λopt (i, j) is calculated r theprocedure moves to step S1018. When the optimal wavelength λopt(i, j)has not yet been calculated, the same default wavelength (for example,λ1) is selected for all the measuring points KP (step 1016), and theprocedure moves to step S1018.

The step S1018 sets the wavelength of the light used by the focus/tiltdetecting system 350 to illuminate the measuring point KP to the optimalwavelength λopt(i, j) or the default wavelength. Thereafter, theexposure shot is set to the exposure position by driving the wafer stage345 (step S1020), and the surface positions by the focus/tilt detectingsystem 350 are measured for the exposure shot by using the optimalwavelength λopt(i, j) or the default wavelength (step S1022).

Next, the wafer 340 is driven based on the surface position measured instep S1022, the focus and tilt are corrected (step S1024), and thepattern on the reticle 320 is exposed onto the wafer 340 (step S1026).It is then determined whether there is a shot to be exposed (orunexposed shot) (step S1028), and the procedure up to step S1020 repeatsuntil there is no unexposed shot. When all the shots are exposed, thewafer 340 is collected (step S1030) and the procedure ends.

The exposure method using the exposure apparatus 301 realizes high focuscorrection accuracy, and improves the resolution and yield even if theDOF is small.

It is an effective optimization of the wavelength of the light used bythe focus/tilt detecting system 350 to illuminate the measuringpositions KP that the optimal wavelength is defined as a wavelength thatprovides a sufficiently symmetrical signal waveform detected by thefocus/tilt detecting system 350. For example, in illuminating the devicepattern that includes, on the wafer 340, a repetitive pattern similar tothe grating pattern 352Cb of the pattern plate 352C shown in FIG. 29,the signal waveforms of all the grating patterns 352Cb deform similarlyand the grating intervals L1, L2 and L3 do not scatter relative to anywavelengths. In this case, the inventive effect is obtained by selectinga wavelength that provides a sufficiently symmetrical waveform as anoptimal wavelength.

As shown in FIG. 41, the light source 352A can be replaced with abroadband light source 352AA, such as a halogen lamp. The broadbandlight source 352AA enables the waveform signal for each wavelength to bedetected, when the dichroic mirrors 354Cb and 354Cc in the detector 354enable the light receiving elements 354Da, 354Db and 354Dc to receivethe lights having central wavelengths λ1, λ2 and λ3. Here, FIG. 41 is ablock diagram showing a structure of a variation of the focus/tiltdetecting system 350.

A description will now be given of a method for selecting the optimalwavelength of light for illuminating the measuring points KP when thepattern plate 352CA shown in FIG. 42 is used instead of the pattern 352Cfor the focus/tilt detecting system 350. FIG. 42 is a plane view showingthe pattern plate 352CA as a variation of the pattern plate 352C.

Referring to FIG. 42, the pattern plate 352CA has one rectangularpattern 352CAb as a light shielding part on a transmission area 352CAaas a light transmitting part. Since the pattern plate 352CA has asmaller projected image area on the wafer 340 than the pattern plate352C, the surface positions on the wafer 340 can be measured at finerpitches. On the other hand, since the grating interval of therectangular pattern 352CAb cannot be measured, another method isnecessary for selecting the optimal wavelength of the light forilluminating the measuring points KP, which is different from the abovemethod.

FIG. 43 is a flowchart for explaining an optimal wavelength selectingmethod of light irradiated by the focus/tilt detecting system 350 whenthe pattern plate 352CA is used. First, surface positions of all thefourteen sample shots are measured with lights having all the differentwavelengths possessed by the focus/tilt detecting system in the shotlayout on the wafer 340 shown in FIG. 35 and a signal waveform isobtained (step S702) FIG. 44 shows one exemplary signal waveformdetected by the detector 354 when the rectangular pattern 352CAb of thepattern plate 352CA is used.

Next, the signal contrast C=(It−Ie)/(It+Ie) is calculated based on thesignal waveform measured by the focus/tilt detecting system 350, where“It” is the light intensity of the transmitting area 352CRa of thepattern plate 352CA and “Ie” is the light intensity of the rectangularpattern 352CAb (step S704) The signal contrast Csij(λ) are calculatedfor all the wavelengths of lights and all the measuring points KP. Here,the suffix “s” is a sample shot number and S=1 to 14. The suffix “i” isan in-shot measuring position number in a slit longitudinal direction,and i=1 to 3. The suffix “j” is an in-shot measuring position number ina scan direction, and j=1 to 7. λ is a wavelength of the illuminationlight, and λ=λ1 to λ3.

Next, an average of the signal contrasts for all the sample shots iscalculated based on the signal contrast Csij(λ), and the wavelength λthat maximizes the signal contrast in the measuring positions (i, j) isdefined as the optimal wavelength λopt(i, j) of each measuring position(i, j) in the shot (step S706). Since the light intensity Itcorresponding to the transmitting area 352CAa of the pattern plate 352CAdepends upon the reflectance of the wafer 340 surface, the signalcontrast C becomes large as the reflectance becomes high. As shown inFIG. 33, the signal waveform that provides a higher reflectance on thewafer 340 causes smaller fluctuations of the reflectance to the coatingthickness variance of the resist 342, reduces influence of distortion ofthe signal waveform, and provides good measurement reproducibility.

After the optimal wavelength λopt(i, j) is selected, the position X1 ofthe image of the rectangular pattern 352CAb is calculated from thesignal waveform of the pattern plate 352CA illuminated by the optimalwavelength opt (i, j), and the surface positions of the wafer 340 aremeasured.

A wavelength that maximizes the reflectance of the light that passes thetransmitting area 352CAa of the pattern plate 352CA may be defined asthe optimal wavelength instead of defining the maximum value of thesignal contrast ratio as the optima wavelength. In this case, in orderto calculate the reflectance, the incident intensity of the illuminationlight is necessary. Therefore, the reference mark table whose spectralreflectance Rref(λ) relative to the illumination light's wavelength isknown is installed on the wafer stage, and the area that has no patternon the reference mark table is measured by the focus/tilt detectingsystem 350, and the signal intensity Iref(λ) of the transmitting area352CAa of the pattern plate 352CA is previously measured.

The reflectance R(λ) at each measuring point KP is calculated byR(λ)=Isin(λ)×Rref(λ)/Iref(λ), where Isin(λ) is the signal intensity ofthe transmitting area 352CAa when the surface positions are measured onthe wafer 340.

The reflectance Rsij (λ) is calculated for all the wavelengths of lightsand all the measuring points KP. Here, the suffix “s” is a sample shotnumber and S=1 to 14. The suffix “i” is an in-shot measuring positionnumber in a slit longitudinal direction, and i=1 to 3. The suffix “j” isan in-shot measuring position number in a scan direction, and j=1 to 7.λ is a wavelength of the illumination light, and λ=λ1 to λ3.

The optimal wavelength λopt(i, j) of each measuring position (i, j) inthe shot may be determined by calculating an average of all the sampleshots based on the reflectance Rsij (A).

The pattern plate 352CA may be replaced with a pattern plate 352CBhaving a circular opening 352CBb in a light shielding area 352Cba asshown in FIG. 45. FIGS. 46A and 46B show a signal waveform detected bythe detector 354 in the focus/tilt detecting system 350 when the opening352CBb of the pattern plate 352CB is used. FIG. 46A shows a sufficientlysymmetric signal waveform, whereas FIG. 46B shows an asymmetric signalwaveform. Here, FIG. 45 is a plane view showing the pattern plate 352CBas a variation of the pattern plate 352CA.

Instead of using the pattern plate 352CA or 352CB, an alternativeembodiment arranges an LED's exit part at a position conjugate with thewafer 340, projects the light emitted from the LED's exit part onto thewafer 340, and receives a projected image through a sensor.

Referring now to FIG. 47, a description will be given of a selectingmethod different from the optimal wavelength selecting method shown inFIG. 38. Here, FIG. 47 is a flowchart for explaining the optimalwavelength selecting method of the light irradiated by the focus/tiltdetecting system 350.

Surface positions of all the fourteen sample shots are measured withlights having all the different wavelengths possessed by the focus/tiltdetecting system 350 in the shot layout on the wafer 340 shown in FIG.35 (step S802), and surface position measurement values Zsij(λ) areobtained for each sample shot, in-shot measuring position, andwavelength of the light used for the measurement (step S804). Here, thesuffix “s” is a sample shot number and S=1 to 14. The suffix “i” is anin-shot measuring position number in a slit longitudinal direction, andi=1 to 3. The suffix “j” is an in-shot measuring position number in ascan direction, and j=1 to 7. λ is a wavelength of the illuminationlight, and λ=λ1 to λ3.

Next, an average value Zasij among wavelengths of the surface positionmeasurement values Zsij(λ) are calculated (step S806). The surface shapeof the wafer 340 (or an approximate curved surface of the surface shape)is calculated using the average value Zasij and curved surface fitting(step S808). FIG. 48 shows the approximate curved surface of the A-A′surface of the wafer 340 shown in FIG. 35, and a graph that plotssurface position measurement values relative to the sample shots L, Dand R for each wavelength.

Next, an offset amount dzsij(λ) of the surface position measurementvalues Zsij(λ) from the approximate curved surface of the wafer 340calculated by the step S808 is calculated (step S810), and a scatteringamount (or standard deviation value) λdZij(λ) among sample shots iscalculated for each measuring point in the shot (step S812) A wavelengththat minimizes the scattering amount (or standard deviation value) σdZij(λ) is defined as the optimal wavelength λopt(i, j) (step S814) In otherwords, the optimal wavelength can be selected using the measurementvalues of the surface positions of the wafer 340 for each wavelength,instead of the feature of the signal waveform.

As discussed, in the exposure, the exposure apparatus 301 uses thefocus/tilt detecting system 350, etc. to position the wafer 340'ssurface that is to be exposed, at the best focus surface, and canprovide devices (such as a semiconductor device, an LCD device, an imagepickup device (such as a CCD), and a thin-film magnetic head) with highthroughput and economical efficiency.

Thus, the present invention can provide a position detecting method andapparatus and an exposure apparatus, which reduce the influence causedby the asymmetry of the alignment mark and provide highly preciseposition detections. In addition, the present invention provides anexposure method and apparatus, which realize a high focus accuracyrelative to a small DOF, and improve the yield.

Further, the present invention is not limited to these preferredembodiments, and various variations and modifications may be madewithout departing from the scope of the present invention. For example,the present invention is applicable to a reticle detecting means fordetecting a position of the reticle.

This application claims a foreign priority based on Japanese PatentApplications, Publication Nos. 2003-407804, filed Dec. 5, 2003, and2004-112535, filed Apr. 6, 2004, each of which is hereby incorporated byreference herein.

1. A wavelength selecting method for selecting a wavelength of light,the light being used to detect a position of a target with a signal froman image of an alignment mark covered with resist, said wavelengthselecting method comprising the steps of: obtaining a reflectance of theresist at a position outside the alignment mark by irradiating lightshaving plural wavelengths to the resist at the position; and selectingone of the lights which one has a wavelength that provides the maximumvalue of reflectance among the reflectances measured by said measuringstep or which one has a wavelength that falls within a predeterminedwave range centering on the wavelength that provide the maximumreflectance.
 2. A wavelength selecting method according to claim 1,wherein said obtaining step includes: detecting light reflected from thealignment mark; and calculating the reflectance of the resist at theposition outside the alignment mark based on the amount of the detectedlight.
 3. A wavelength selecting method according to claim 2, whereinsaid calculating step calculates the reflectance from an opticalconstant and coating thickness of the resist at the position outside thealignment mark.
 4. A wavelength selecting method according to claim 1,wherein said selecting step selects light having a wavelength λ thatmeets the following equations, where n1 and d1 are a refractive indexand a thickness of the resist at the position, n2 is a refractive indexof the target, and m is 0 or a positive integer:if n1<n2, then n 1 d 1=λ/2+mλ/2; andif n1>n2, then n 1 d 1=λ/4+mλ/2.
 5. A wavelength selecting methodaccording to claim 1, wherein said selecting step selects light having awavelength λ that maximizes a reflectance R obtained by the followingequation, where n1 and d1 are a refractive index and a thickness of theresist at the position, θi is an incident angle from the resist to thetarget, and J is a natural number: $\begin{matrix}{{Ri} = {1 - \frac{4n\quad 1^{2}{n2}}{{{n1}^{2}\left( {1 + {n2}} \right)}^{2} + {\left( {{n2}^{2} - {n1}^{2}} \right)\left( {1 - {n1}^{2}} \right)\sin^{2}\delta\quad i}}}} \\{{\delta\quad i} = {\frac{2\pi}{\lambda}{n1d1}\quad\cos\quad\theta\quad i}} \\{R = {\frac{1}{J}{\sum\limits_{i = 1}^{J}\quad{Ri}}}}\end{matrix}$
 6. A wavelength selecting method according to claim 1,wherein the predetermined range is a wave range of 30 nm centering onthe wavelength that maximizes the reflectance.
 7. A wavelength selectingmethod for selecting a wavelength of light, the light being used todetect a position of a target with a signal from an image of analignment mark covered with resist, said wavelength selecting methodcomprising the steps of: previously storing, for each optical constantand coating thickness of the resist, a wavelength of light thatmaximizes a reflectance of the resist at a position outside thealignment mark; obtaining the optical constant and coating thickness ofthe resist; and selecting one of the wavelengths which one maximizes thereflectance of the resist and corresponds to the optical constant andcoating thickness of the resist obtained by said obtaining step.
 8. Aposition detecting method for detecting a position of a target using asignal from an image of an alignment mark coated with a resist, saidposition detecting method comprising the steps of: irradiating analignment mark using light having a wavelength selected by using awavelength selecting method according to claim 1; and generating thesignal from the light reflected from the alignment mark irradiated bysaid irradiating step.
 9. A position detecting apparatus for detecting aposition of a target using a signal from an image of an alignment markformed on the target that is coated with resist, said position detectingapparatus comprising: a selector for selecting the light having awavelength that maximizes a reflectance of the resist at the positionoutside the alignment mark; and a signal processor for determining aposition of the alignment mark relative to the signal generated from thelight having the wavelength selected by said selector.
 10. An exposureapparatus for exposing a pattern on a reticle onto an object via aprojection optical system, said exposure apparatus comprising a positiondetecting apparatus according claim 9, and using the position detectingapparatus for alignment of the object.
 11. An exposure method forexposing a pattern on a reticle onto an object via a projection opticalsystem, said exposure method comprising the steps of: aligning theobject using light having a wavelength selected using a wavelengthselecting method according to claim 1; and projecting the pattern ontothe object that has been aligned.
 12. A device manufacturing methodcomprising the steps of: exposing a pattern on a reticle onto an objectusing an exposure apparatus; and developing the object that has beenexposed, wherein said exposure apparatus includes a position detectingapparatus according claim 9, and using the position detecting apparatusfor alignment of the object.
 13. An exposure apparatus for exposing apattern on a reticle onto an object via a projection optical system,said exposure apparatus comprising: an irradiation unit for irradiatingplural lights having different wavelengths onto two or more measuringpoints in each of the plural shots; a detector for detecting reflectedlight from the measuring points; a selector for selecting one of thewavelengths in accordance with the measuring positions of the pluralshorts based on a detection result from said detector; and a calculatorfor calculating a position of each of the plural shots in anoptical-axis direction based on the one of the wavelengths selected bysaid selector.
 14. An exposure apparatus according to claim 13, whereinsaid irradiation unit includes: a pattern plate that has a gratingpattern having plural elements; and a projection unit for projecting animage of the grating pattern to plural shots, and wherein said selectorselects one of the wavelengths which one minimizes scattering amongplural shots, with respect to plural element intervals of an image ofthe grating pattern.
 15. An exposure apparatus according to claim 13,wherein said irradiation unit includes a projection unit for projectingan image of the grating pattern to plural shots, and wherein saidselector selects one of the wavelengths which one provides a maximumcontrast of an image of the predetermined image.
 16. An exposureapparatus according to claim 13, wherein said irradiation unit includes:a pattern plate that has a transmitting part that transmits light, and ashielding part that shields the light; and a projection unit forprojecting an image of the grating pattern to plural shots, and whereinsaid selector selects one of the wavelengths which one provides amaximum reflectance of light that passes through the transmitting partin the pattern plate.
 17. An exposure apparatus according to claim 13,wherein said selector selects one of the wavelengths which one minimizesscattering among plural shots, with respect to positions in theoptical-axis direction of plural shots calculated from the plural lightshaving different wavelengths.
 18. A wavelength selecting method forselecting a wavelength of light used to detect a position of a target,said wavelength selecting method comprising the steps of: projecting animage of a pattern that includes plural elements using plural lightshaving different wavelengths onto measuring positions on plural areas ofthe target, and detecting a signal waveform from plural areas of thetarget; obtaining plural element intervals for each of the pluralwavelengths based on the signal waveform detected by said detectingstep; calculating a standard deviation of the plural element intervalsat the measuring positions in the plural areas based on the pluralelement intervals; and selecting one of the wavelengths which oneprovides a minimum standard distribution calculated by said calculatingstep.
 19. A wavelength selecting method for selecting a wavelength oflight used to detect a position of a target, said wavelength selectingmethod comprising the steps of: projecting an image of a pattern thatincludes plural elements using plural lights having differentwavelengths onto measuring positions on plural areas of the target, anddetecting a signal waveform from plural areas of the target; obtaining asignal contrast of the signal waveform for each of the pluralwavelengths based on the signal waveform detected by said detectingstep; and selecting one of the wavelengths which one provides a maximumcontrast of the signal waveform for each of the plural wavelengths basedon the signal waveform detected by said detecting step.
 20. A wavelengthselecting method for selecting a wavelength of light used to detect aposition of a target, said wavelength selecting method comprising thesteps of: projecting an image of a pattern that includes plural elementsusing plural lights having different wavelengths onto measuringpositions on plural areas of the target, and detecting a signal waveformfrom plural areas of the target; calculating a reflectance at themeasuring position of light that passes the element based on the signalwaveform detected by said detecting step; and selecting one of thewavelengths which one provides a maximum reflectance of the signalwaveform for each of the plural wavelengths based on the signal waveformdetected by said detecting step.
 21. A wavelength selecting method forselecting a wavelength of light used to detect a position of a target,said wavelength selecting method comprising the steps of: projecting animage of a pattern that includes plural elements using plural lightshaving different wavelengths onto measuring positions on plural areas ofthe target, and obtaining positional information indicative of theposition; obtaining an approximate curved surface from an average of thewavelengths of the positional information obtained in said obtainingstep; calculating an offset amount of the positional information fromthe approximate curved surface; and selecting one of the wavelengthswhich one minimizes scattering among the plural areas based on theoffset amounts calculated by said calculating step.
 22. An exposuremethod for exposing a pattern on a reticle onto plural shots on anobject through a projection optical system, said exposure methodcomprising the steps of: detecting a position of the object in anoptical-axis direction using light having a wavelength selected by awavelength selecting method according to claim 18; and scanning theobject in synchronization with the reticle, based on the position of theobject in the optical-axis direction detected by said detecting step.23. An exposure method for exposing a pattern on a reticle onto pluralshots on an object through a projection optical system, said exposuremethod comprising the steps of: detecting a position of the reticle inan optical-axis direction using light having a wavelength selected by awavelength selecting method according to claim 18; and scanning thereticle in synchronization with the object, based on the position of thereticle in the optical-axis direction detected by said detecting step.24. A device manufacturing method comprising the steps of: exposing anobject using an exposure apparatus according to claim 13; and developingthe object that has been exposed.